Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!pasteur!ucbvax!decwrl!labrea!rutgers!att!mtunb!dmt
From: dmt@mtunb.ATT.COM (Dave Tutelman)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Major Modem Woe: THE 9600B modem no good for interactive use
Summary: Sorry, but that's what half-duplex means.
Message-ID: <1407@mtunb.ATT.COM>
Date: 17 Feb 89 21:47:16 GMT
References: <7298@june.cs.washington.edu>
Reply-To: dmt@mtunb.UUCP (Dave Tutelman)
Organization: AT&T Bell Labs - Lincroft, NJ
Lines: 88

In article <7298@june.cs.washington.edu> robertb@uw-june (Robert Bedichek) writes:
>I just bought a pair of THE(r) 9600 baud modems and now find that
>apparently they are unsuitable for interactive use at speeds above 2400
>baud!
>
>The problem is that they are half duplex and the modem takes a full
 			      ^^^^^^^^^^^
>second to turn the line around.  
	That is what half-duplex means; it's that simple.  If the modem
	was advertised as half-duplex, you have only yourself to blame.
	I recognize that not everyone is a modem techie, BUT you ought
	to know what the buzzwords mean in the specs, when you buy
	something.

>The result is that response to key strokes is delayed by a full second.
	I know it's no consolation but "that's just the way they work."

>I find it hard to believe that anyone would design a modem like the THE
>9600, which is meant to be low cost ($400 apiece) and plug into PC's,
>and make it not usable for 99 percent of the applications people with
>PC's put their modems to.
	Half-duplex modems are intended for bulk data transfers, using
	block sizes large enough so that the line turn-around time
	for acknowledgment doesn't negate the line speed.  If two line
	turn-arounds take 1 sec (probably a reasonable number, and in
	agreement with your observation above) then the block size
	should be well over 9600 bits (1000 bytes).

	Certainly not efficient for interactive use, as you've
	noticed :-(, but certainly not useless for PCs.  YOU use YOUR
	modem for interactive communications, but lots of people only
	use their modem to download stuff from BBSs.  This modem is
	ideal for them.

>I realize that the hardware to do 9600 in both directions at the same
>time is expensive,
	There IS a way that's probably cheaper than the modem you bought, 
	but you need TWO lines.  That's either two simultaneous phone 
	calls, or a "four-wire private line".  Going 9600 bits per second
	full duplex on an average dial-up connection is a real trick.

>but what's expensive about having the line be able
>to switch directions in, say, 50 milliseconds (assuming a short-haul)?
	...and how does the modem know it's short-haul?  And why should
	that make a difference?  The turn-around delay isn't a function
	of how fast the signal travels; it's how fast the modem can
	reliably detect and synch to the signal.

>Or having a low speed channel going the other way (I think that the
>Telebit Trailblaisers have a 75 baud channel going the opposite
>direction of the high speed channel)?
	...so why didn't you buy a Telebit?
	Seriously, all these things are feasible but cost money.
	Someone aiming at a low-end PC-market 9600-baud modem is
	going to build a feasible equalizer/mod/demod, and spend
	NOTHING on the frills you suggest.

>Am I really stuck?  PC Network is making noises like I can't return
>these modems for cash, and perhaps not every for an in-store credit!
	I have no personal experience with PC Network,
	having been scared away by lots
	of previous postings on the net.  But I'm sure you'll get
	suggestions from those who have dealt with them.

>Can anyone recommend a 9600 baud modem that doesn't have this problem.
>I'm thinking about getting the USR 9600 HST, which are $225 more per
>modem, but which at least *work*.
	Is it Half-Duplex or Full-Duplex?  If full-duplex, can it work
	over a single dial-up connection?  If Half-duplex, what's its
	line-turn-around time?  Don't buy it without being satisfied
	with the answers to these questions.

>Rob "I'm-really-bummed" Bedichek

Rob, I'm sorry that you're bummed, and I'm sorry that I can't sound
more sympathetic with your plight.  I'm not angry with you, and I DO
feel sorry for your predicament.  But:
   -	Half-duplex has a meaning, it's an old and well-understood
	meaning, and you've just discovered what it is.
   -	The modem you bought is not a stupid product, as you intimated.
	It's just stupid for your intended application.

+---------------------------------------------------------------+
|    Dave Tutelman						|
|    Physical - AT&T Bell Labs  -  Lincroft, NJ			|
|    Logical -  ...att!mtunb!dmt				|
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